Mr. B. T. Grlazebrook on Nicol's Prism. 



251 



Substituting the values from above, we find 



= 11° 75% . . . 

 and 



<r = 5°3'. . . . 



(9) 



(10) 



Thus in this case the plane of polarization of the emergent 

 light is inclined to a plane fixed in the Nicol at an angle of 5° 3'. 



Let Ox, Oy, Oz be the three rectangular axes taken above, 

 viz. the normal to the face of incidence drawn inwards, and 

 two lines parallel to AC and BD respectively. 



Let N in the plane x y be inclined at 20° to x. ON 



is approximately parallel to an edge of the rhomb, and is the 

 axis round which the rhomb is turned. 



Let N' in the plane N Z be inclined at 5° to K N' 

 is the direction of the incident or emergent-wave normal 

 in the case considered above ; and the plane of polarization 

 of the emergent wave is inclined at an angle of 5° 3 / to the 

 plane xOy. 



Now let us suppose the Nicol rotated through an angle of 

 90° about ON. Let Ox*, Oy f , Ozf be the new positions of the 

 axes ; then ON' lies in the plane y'Ox'. 



^0^=25°, N x (y = 65 , NW=90°. 



Thus, if I', m f , nf are the new direction-cosines of ON 7 , 



V= cos 25°, m'= cos 65°, n'=0. 



And in this case the intersection of the incident wave and the 

 face of incidence is a possible direction of vibration ; for it is 

 parallel to the line BD. Also using the same notation as pre- 

 viously, yjr, ^, 6, 6' ', and a are all zero. 



The plane of polarization of the emergent wave coincides 



T2 



